Electrical recording medium



United States Patent ELECTRICAL RECURDING MEDIUM Application November 3,1953, Serial No. 390,096

13 Claims. (Cl. 204-4) No Drawing.

This invention relates to recording media of the type which aresubjected to an external application of heat to render the mediummarkable electrically.

While not limited thereto, the invention is particularly useful in theart or electrical recording wherein it is desirable to pass a recordingcurrent through a conductive record medium containing anelectro-responsive marking compound. in recorders operating on thisprinciple, the recording current is applied to two cooperatingelectrodes, and a conductive sheet or tape, which may be impregnatedpaper or cloth, is drawn between the electrodes. Marks are made on theconductive medium by electrochemical change.

Paper, being white and inexpensive, is very desirable for recordpurposes. However, paper is normally nonconductive and must beimpregnated with suitable chemicals to make it electro-conductive andsuitable for use in electrical recorders. Perhaps the most successful ofthe recording papers in use at the present time are the typesimpregnated with aqueous electrolytes and a marking compound such asdisclosed in patents to J. V. L. Hogan et all, 2,339,267, and E. R.Wagner, 2,358,839. These papers are eminently suitable for mostrecording applications but have the characteristic that they must bemaintained with the proper moisture content during storage untilactually recorded on by recording apparatus.

Another type of electro-graphic recording paper is known and isdisclosed in patent to R. B. Gibney, 2,530,956. This type of paper isfabricated as a composite sheet of several layers. One layer, therecording layer, is impregnated with a solid compound consisting ofacetamide, alone or in combination with propionamide or formamide.Another layer is impregnated with the same amide or amides and anionizable organic dye, the ions of which migrate to the recording layerwhen the solid impregnant is melted by the passage of an electriccurrent therethrough, to produce a characteristic mark. In recordingpaper of this type there is the disadvantage that at least two layersare required to form the composite sheet. The acetamide which forms thebasic impregnant sublimes or evaporates readily at normal ambienttemperatures and pressures so that the paper must be stored in a sealedcontainer to remain operative.

In moist types of recording papers such as disclosed in Patent 2,358,839mentioned above, and in patent to Solomon 2,419,296, the papers areimpregnated with a variety of marking compounds and may include asoptional ingredients small quantities of urea or thiourea asantioxidants or color intensifiers. Quite unexpectedly, applicantdiscovered that it is possible to provide a wholly dry, white recordingmedium employing only a single layer sheetor tape of paper or clothimpregnated with only urea and/of thiourea and an electro-responsivesubstantially colorless marking compound. An ionizable salt may be addedto the impregnant for a purpose to be described. The recording medium ismarkable electrically only by being first heated until the impregnant ismolten,

and the medium is then subjected to a suitable electrical voltagebetween electrodes.

2,776,231 Fatented Jan. 1, 1957 ice The recording medium ischaracterized by its extreme stability when stored under varyingconditions of temperature and humidity and prolonged exposure to theambient atmosphere.

It is therefore a principal object of the invention to provide a dryrecording medium which can be rendered conductive only by heating and isthen electrically markable.

It is a further object to provide a white, dry, single layer porousrecording medium impregnated with urea and/ or thiourea, with or withoutan ionizable salt, and with a substantially colorless electro-responsivemarking compound.

It is a further object to provide a dry recording medium containing animpregnant which medium is electroconductive and electrically markableonly when heated to at least the melting point of the impregnant, andwhich retains its markability on prolonged exposure to varying ambientatmospheric conditions.

It is a further object to provide a dry recording medium impregnatedwith a solid material which does not require special packaging forstorage prior to use.

It is a further object to provide a novel impregnant composition for arecording medium.

It is a further object to provide a recording medium which may beemployed in an intermittently operated recorder without portions of themedium which are exposed cluring the idle period becoming inoperative.

It is a further object to provide a recording medium which is dry, inthe sense of being solid as well as nonaqueous, prior to and subsequentto marking.

For a description of a recording apparatus and method suitable for therecording medium of this invention reference is had to my Patent2,662,803.

It is thus far apparent that the dry recording paper of the presentinvention is radically dilierent from known dry recording papers of thetype which involve a carbonized body and a thin gray coating which isblasted olf in the recording process. The present paper is whitethroughout and it permits of recordings including the white end of thetone scale rather than being limited to a tone scale from gray to black.The present paper is also relatively low in cost by reason ofutilizingstandard paper stock, and is relatively non-inflammable.

In addition to urea and/ or thiourea, the normally solid impregnant mayinclude a highly ionizable salt to increase conductivity of theimpregnant when in the molten condition. Any of numerous highlyionizable salts can be used such as, for example, sodium nitrate,potassium nitrate, ammonium nitrate, ammonium chloride, ammoniumsulfate, and sodium chloride. Recitation of these examples of such saltsis not to be taken as limiting the invention, which is applicablegenerally to all highly iomzable salts both inorganic and organic.

It is to be emphasized that the normal functioning of the impregnant ofthis invention does not require the presence of moisture to any degreewhatever. On the contrary the recording paper containing the impregnantis useful in the Wholly bone dry state since it only requires theapplication of sufiicient heat to melt the impregnant to render therecording paper electrically markable.

The impregnant may be formed by melting urea and/ or thiourea and addinga relatively small amount of the salt. Or the urea and/ or thiourea andthe salt may be dissolved in an appropriate common solvent which maythen be removed by evaporation leaving the dry, white impregnant as anelectrolyte.

In making a normallydry recording medium which, at elevatedtemperatures, is markable responsively to electric currents, a suitablewhite substantially colorless marking compound is added with the salt tothe molten urea or urea and/or thiourea. Electro-responsive markingcompounds fall into at least two categories: those depending onelectro-chemical change, an example of which is catechol; and thosedepending on thermo-chemical change, an example of which is leadthiosulfate. Catechol is normally a white crystalline compound whichturns black when in an electrolyte subject to electric current from aniron electrode having a positive polarity. Lead thiosulfate is alsowhite and it turns black when heated to about 200 deg. C. This heat maybe provided in an elemental area by the passage of current through anelectrolyte containing the lead thiosulfate, and therefore it properlymay be called an electro-responsive marking compound. It will beunderstood that when a thermochemical type marking compound is employedin a normally-solid electrolyte, the electrolyte is rendered molten andthus conductive by the application of an appropriate amount of heat, andthen a marking current is passed through the molten electrolyte with theresult that additional heat is generated locally which turns thelocallypresent lead thiosulfate black. The heat below lSO'deg. C.applied to melt the electrolyte does not in any way affect the color ofthe lead thiosulfate.

As an example of the manner in which a sheet of recording papercontaining urea and an ionizable salt is rendered electro-conductivewhen heated, the following experiments may be performed.

TEST I About 24 g. of urea and 3 g. of potassium nitrate are added to 20milliliters of water and heated to boiling. A piece of absorbent paperstock is immersed in the hot solution, removed, and allowed to cool anddry. The resistance of the dry impregnated paper as then measured byapplying two probes of an ohmmeter to opposite sides of the paper willbe found to exceed one megohm, so that the paper may be consideredsubstantially non-conductive. If the paper is now placed on a hot platehaving a temperature of about 120 deg. C., the paper will assume a wetappearance and have an electrical resistance in the order of from 500 to20,000 ohms depending on the contact pressure and spacing on one side ofthe paper of the two ohmmeter probes. The paper remains conductive aslong as it is kept-in a heated state. When the paper is allowed to cool,the electrical resistance again exceeds one megohm.

Test I may be performed with the impregnant consisting of thiourea andan ionizable salt or with a mixture of urea and thiourea and anionizable salt. The electrical resistance of the paper when theimpregnant is molten will generally be found to be highest when thioureais used without urea. Mixtures of urea and thiourea will generallyproduce resistances intermediate in value between urea used alone andthiourea used alone. In general higher temperatures will be required tomelt the impregnant as the proportion of thiourea is increased. Test Imay be performed without the addition of potassium nitrate or any otherionizable salt, so that the impregnant consists of urea and/or thioureaalone. The range of resistances of the paper when the severalimpregnants are molten will generally be higher than when the ionizablesalt is used, with the highest resistance obtained by use of thioureaalone. The reason that the test can be successfully performed withoutuse of an ionizable salt is believed to be that urea and/ or thioureawhen heated until molten break down in such a manner as to create freeions which render the paper support electroconductive.

Advantage is taken of the electroconductive characteristics of urea and/or thiourea when molten in the present invention. The following examplesare intended as illustrative of ways in which the invention can bedemonstrated:

Example I About 8 g. of urea and l g. of potassium nitrate are added to10 milliliters of water and heated to boiling. About 2 g. of leadthiosulfate is added to the water solution. Lead thiosulfate is athermo-chemical type of marking compound. A piece of absorbent paperstock is immersed in the hot solution including dispersed leadthiosulfate, removed, and allowed to dry. The dried paper is then placedon a hot plate having a temperature of about deg. C., whereupon it willassume a wet appearance. One terminal of a 45-volt dry battery is thenconnected to the metallic hot plate and the other electrode connected toa stylus. When the stylus is touched to or drawn over the paper, anintense black mark is left on the paper. The electric current passingthrough the paper generates heat locally to transform the leadthiosulfate into a black compound. Electrical conductivity by meltingthe electrolyte in the paper is, of course, a prerequisite to thepassage therethrough of marking current.

A suitable recording medium can be obtained by use of a mixture ofthiourea and urea or of thiourea alone in accordance with Example Iinstead of urea alone. The relative proportions of urea and thiourea inthe mixture if used have not been found to be critical. The intensity ofthe black mark will generally be less as the proportion of thiourea isincreased because the higher resistance of the molten impregnant resultsin a weaker marking current. Also higher temperatures will generally berequired to melt the impregnants containing thiourea.

Example II About 20 g. of urea is melted at about deg. C., with 0.3 g.of catechol. Paper impregnated with this solution has a slight pinkishtinge and when heated until the impregnant is molten has a resistance ofabout l00l0,000 ohms, measured as described in Test I. It will be notedfrom the above that no ionizable salt is added to the impregnant. Whencurrent is passed through the heated paper from an iron stylus connectedto the positive terminal of a 45-volt battery to a hot plate contactingthe paper and connected to the negative terminal of the battery, thestylus traces a dark black mark on the paper.

Example II brings out the fact that urea is sufficientlyelectroconductive in the molten state without the addition of a highlyionizable salt to render the recording medium electrically markable. Itwill be noted that no water is employed in the impregnant of Example II.If desired the urea and catechol may be dissolved in about 30milliliters of water and heated to boiling to form the impregnant.

A suitable recording paper may be obtained by employing a mixture ofurea with thiourea or by employing thiourea alone in accordance withExample II instead of urea alone. The relative proportions of urea andthiourea in the mixture if used have not been found to be critical. Themarking currents will generally be weaker because of the increased rangeof resistance of the molten impregnants containing thiourea. Also highertemperatures Will be required to melt the impregnants containingthiourea. To obtain an intensity of mark closer to that obtained by useof urea and catechol alone, a higher voltage must be applied.

The present invention thus provides a means of control in thefabrication of the recording medium to adapt it for use in recordersoperated at temperatures considerably above the melting point of urea.Addition of thiourea to the urea or substitution of thiourea for theurea will result in a recording medium in which the melting point of theimpregnant is closer to that of the operating temperature of therecorder. Furthermore, the voltages available in the recorder may beexcessively high for a recording paper containing urea and an ionizablesalt. By omitting the salt or reducing the quantity used, and by addingthiourea to the urea or using it instead of the urea, the electricalresistance of the recording medium can be accommodated to the particularmarking voltages available at the recorder.

The several examples described above are not to be construed as limitingthe invention. It will of course be understood that the recording paper,when manufactured in quantity may be made continuously by passing thepaper stock through an impregnating bath and drier.

One of the important advantages of a recording medium constructed inaccordance with the teaching of this invention is that the same sheetmay be heated and recorded on any number of times. Expressed anotherway, the medium is electrically markable when heated to the meltingpoint or" the impregnant regardless of having been so heated previously.The sensitivity of the medium is not destroyed by a developing or fixingprocess. Yet each recorded mark is permanently fixed in the impregnatedfibers of the support at the instant that the mark is made. Thecontinued sensitivity of the recording medium makes it valuable forspecial applications such as in the instrumentation field where it maybe desirable to make a comparison record of values taken at one timewith values taken at a previous time.

In order to demonstrate the stability of recording paper embodying theinvention, samples of dry recording paper were made employingimpregnants containing acetamide and propionamide which are amides whoseuse in recording media is taught in the prior art. Urea and thiourea mayalso be considered as amides since both compounds are amide derivativesof carbonic acid. It was found that recording media containing theseprior art amides were unstable, or had other objectionablecharacteristics as discovered in the tests described below. Samples ofimpregnated recording paper prepared as described in Example I wereformulated as follows:

SAMPLE A Actamide, 20 g. Potassium nitrate, 0.3 g. Catechol, 0.3 g.

SAMPLE B Acetamide, 20 g. Propionamide, 2 g. Potassium nitrate, 0.3 g.Catechol, 0.3 g.

In addition, samples of recording paper were prepared as described inExamples 1 and II respectively. The samples each included as anelectrolyte urea, thiourea, or a mixture of urea and thiourea, with andwithout an ionizable salt, in addition to a marking compound.

TEST FOR EFFECTS OF PROLONGED EXPOSURE The several samples of recordingpaper formulated as described above were each heated on a hot plate setat 130 degrees C. and tested for markability at 45 volts.- Black marksof different intensities were produced on all samples when an electrodewas touched to and was drawn over each sample of paper. The severalsamples. were then exposed to normal varying ambient atmosphericconditions for a period of twenty days. Again each sample was heated ona hot plate of 130 degrees C. and tested for markability at 45 volts asabove. Black marks were produced on all paper samples containing ureaand/or thiourea when an electrically positive iron electrode was touchedto and was drawn over the papers. No mark was produced on any of thesample papers containing acetamide. It was found that the amide contentof these papers had sublimed or evaporated to the extent that no markingwas possible when tested for markability at 45 volts in accordance withthe method of Example I.

OBSERVATIONS AND CONCLUSIONS The above test clearly demonstrated thatthe physical condition of the papers containing urea and/or thioureawere substantially unaffected by conditions of prolonged exposure tovarying ambient atmospheric conditions, and their markability, whentested in accordance with Example I by externally heating until theimpregnant was molten and then applying an electrical voltage wasunimpaired. It was furthermore found that the urea and/or thiourea inthese papers sublimed or evaporated to such a small extent under openstorage conditions that the markability of the papers was not impaired.The amide content of the papers containing either acetamide or a mixtureof acetamide with propionamide under conditions of prolonged exposure toambient atmospheric conditions sublimed or evaporated to the extent thatmarking by external heating and application of an electrical potentialin accordance with Example I was not possible.

It must be emphasized that it is essential to the invention that theimpregnant of the recording medium be such that the recording mediumwill remain operative upon prolonged exposure to varying ambientatmospheric conditions. Thus the rate of sublimation or evaporation ofthe impregnant is a critical factor. The rate of sublimation orevaporation is of course directly related to the inherent vaporpressure. It will be noted that urea, thiourea, and mixtures thereof allhave inherent vapor pressures less than acetamide.

The invention has been described by reference to specific examples, butthis has been done by way of explanation and not limitation, and thescope of the invention is to be construed by reference to the appendedclaims.

One particularly important extension of the invention is in connectionwith certain impregnants for recording paper containing the particularamides having inherent vapor pressures less than acetamides disclosed inmy copending applications Ser. No. 350,757, filed April 23, 1953, andSer. No. 355,843, filed May 18, 1953. Among these amides arechloroacetamide, cyanoacetamide and other hydrogen substitution productsof acetamide; also acetanilide, benzamide, paratoluene sulfonamide,N-ethyl sulfonamide, butyramide, and other primary and secondary amideshaving at least four carbon atoms. Urea or thiourea or a mixture of ureaand thiourea may be used to replace part or all of the ionizable saltsused in any of the impregnants including the amides mentioned above,when it is desired to produce a recording medium having a higherresistance when the impregnant is molten than is otherwise obtained byuse of ionizable salts.

This application is a continuation-in-part of my copending applicationSerial No. 102,135, filed June 29, 1949, now abandoned.

What is claimed is:

1. A white dry recording medium comprising a porous sheet impregnatedwith a major quantity of urea and a substantially colorlesselectroresponsive marking compound, said medium being substantiallynon-conductive electrically at temperatures below the melting point ofurea, and having high electrical conductivity and electrical markabilityonly at temperatures above the melting point of urea.

2. A white dry recording medium according to claim 1, wherein theimpregnant further includes thiourea.

3. A white dry recording medium comprising a porous sheet impregnatedwith a major quantity of urea, a highly ionizable salt, and asubstantially colorless electroresponsive marking compound, said mediumbeing substantially non-conductive electrically at temperatures belowthe melting point of urea, and having high electrical conductivity andelectrical markability only at temperatures above the melting point ofurea.

4. A white dry recording medium according to claim 3, wherein theimpregnant further includes thiourea.

5. A white dry recording medium comprising a porous sheet impregnatedwith a major quantity of an amide selected from the group consisting ofurea and thiourea, and a substantially colorless electroresponsivemarking compound, said medium being substantially non-conductiveelectrically at temperatures below the melting point of the amide andhaving high electrical conductivity and electrical markability only attemperatures above the melting point of the amide.

6. A white dry recording medium according to claim 5, wherein theimpregnant further includes an ionizable salt.

7. A white dry recording medium according to claim 5, wherein theimpregnant includes another amide having an inherent vapor pressure lessthan that of acetamide.

8. A white dry recording medium according to claim 7, wherein theimpregnant further includes an ionizable salt.

9. A white dry recording medium according to claim 7, wherein the saidother amide is a hydrogen substitution product of acetamide.

10. A white dry recording medium according to claim 7, wherein the saidother amide is selected from the group consisting of acetanilide,benzamide, para-toluene sulfonamide, N-ethyl sulfonamide, andbutyramide.

References Cited in the file of this patent UNITED STATES PATENTS2,358,839 Wagner Sept. 26, 1944 2,419,296 Solomon Apr. 22, 19472,530,956 Gibney Nov. 21, 1950 pair

5. A WHITE DRY RECORDING MEDIUM COMPRIDING A POROUS SHEET IMPREGNATEDWITH A MAJOR QUANTITY OF AN AMIDE SELECTED FROM THE GROUP CONSISTING OFUREA AND THIOUREA, AND A SUBSTANTIALLY COLORLESS ELECTRORESPONSIVEMARKING COMPOUND, SAID MEDIUM BEING SUBSTANTIALLY NON-CONDUCTIVEELECTRICALLY AT TEMPERATURES BELOW THE MELTINGG POINT OF THE AMIDE ANDHAVING HIGH ELECTRICAL CONDUCTIVITY AND ELECTRICAL MARKABILITY ONLY ATTEMPERATURES ABOVE THE MELTING POINT OF THE AMIDE.